Field of the Invention
The present invention relates to a printing apparatus for performing printing by liquid ink.
Description of the Related Art
There is known an inkjet printing apparatus that includes a sub-tank capable of temporarily storing ink in a channel from an ink tank to a printhead to improve the consumption properties of ink in an ink tank or prevent ink from running out during a printing operation. Even if ink in the ink tank that is a main tank runs out during a printing operation, this printing apparatus can continue the printing operation by using ink in the sub-tank. The remaining ink amount in the main tank can be determined by detecting the remaining ink amount in the sub-tank. To detect the remaining ink amount in a tank, there is generally known a method of detecting the liquid surface height of ink. As such methods, an electrode method and an optical method are known. The electrode method is a method of detecting the liquid surface height of ink based on the electrically connected state between two electrode pins provided in a tank. The optical method is a method of detecting the liquid surface height of ink based on the reflection state of a light beam by a photosensor and a prism.
On the other hand, as a method of supplying ink from the main tank to the printhead via the sub-tank, a method using an ink water head difference is known. If ink is discharged from the nozzles of the printhead by a printing operation or suction operation in a state in which the channel is filled with the ink, air as much as the ink supplied to the printhead flows from an air vent into the main tank. If the ink in the main tank runs out, the ink in the sub-tank is consumed. At this time, bubbles remaining in the main tank or ink channel flow into the sub-tank.
The bubbles may affect detection of the liquid surface height of ink. For example, in the electrode method, when bubbles come into contact with the electrodes, the electrically connected state may be maintained even if the liquid surface lowers, and correct detection may be impossible. In the optical method as well, if bubbles adhere to the prism surface, the light beam may be reflected irrespective of the presence/absence of a liquid, and correct detection may be impossible.
Japanese Patent Laid-Open No. 2007-237552 discloses a structure that divides a sub-tank open to air into two chambers by a partition. The two chambers communicate on the upper and lower sides of the partition. Air can be distributed on the upper side, and ink can be distributed on the lower side. The detecting position of the sensor of the optical method is set in one chamber. The partition traps bubbles, thereby preventing the bubbles from entering the chamber in which the detecting position of the sensor of the optical method is set.
However, the structure of the sub-tank of Japanese Patent Laid-Open No. 2007-237552 is a structure with an air vent. In a sub-tank without an air vent, even if the partition of Japanese Patent Laid-Open No. 2007-237552 is provided, when the liquid surfaces in the two chambers lower, bubbles as much as the lowering of the liquid surfaces are brought into the chamber in which the detecting position of the sensor is set.